1. Field of the Invention
The present invention relates to a method for preparing high-purity elemental phosphorus, in particular, one suitable for use as a raw material for high-purity dry phosphoric acid for semiconductor applications and a method for preparing high-purity phosphoric acid using the same.
2. Background Art
White phosphorus contains traces of metal impurities such as Fe, Sb, As and Zn. These metal impurities are derived from phosphate rock, as a raw material of white phosphorus, and are present as impurities in phosphoric acid when the phosphoric acid is prepared from white phosphorus. In recent years, phosphate rock, one white phosphorus raw material, has undergone deterioration in qualities in terms of a resource. Accordingly, the level of impurities, in particular, arsenic or antimony, present in white phosphorus has increased.
Arsenic in crude phosphoric acid is insolubilized as an arsenic sulfide by blowing hydrogen sulfide gas into crude phosphoric acid and can be removed to a level of 30 ppb or less by filtration. However, when the level of arsenic in crude phosphoric acid increases, it takes a long period of time to filter arsenic sulfide, and problems such as deterioration in the filtration rate caused by clogging of arsenic sulfide or a great deterioration in the operation efficiency of a phosphoric acid purification process due to a complicated exchange operation for furnaces occur.
Antimony in crude phosphoric acid is also insolubilized by blowing hydrogen sulfide gas into crude phosphoric acid and is removed by filtration. However, in the case where a great amount of antimony is present in the white phosphorus, it is often difficult to decrease the amount of antimony to 200 ppb or less.
High-purity phosphoric acid is used as an etching agent for SiN films used for silicon wafers for semiconductor applications. In some cases, it is difficult to reduce the amount of impurities present in high-purity phosphoric acid for semiconductor applications and it is thus required to reduce the amount of antimony to 200 ppb or less.
For this reason, it is industrially advantageous that arsenic and antimony are crushed and removed in a state of white phosphorus as a raw material.
Washing with nitric acid is suggested as a method of removing arsenic from crude white phosphorus (for example, see Japanese Patent Application Laid-open No 49-95891). This method has high arsenic removal efficiency, but disadvantageously entails a decrease of white phosphorus yield to about 50% after washing, since phosphorus is released in nitric acid, and production of a great amount of nitric acid waste containing phosphoric acid.
Another removal method is suggested which includes mixing crude white phosphorus with an iodine compound such as iodine oxide or iodic acid, heating the mixture to convert arsenic in the phosphorous into arsenic oxide and heating the mixture to the boiling point or less of arsenic oxide to recover elemental phosphorus (for example, see Japanese Patent Application Laid-open No 06-40710). However, with this method, it is possible to remove only arsenic and is impossible to remove antimony.
In addition, U.S. Pat. No. 5,989,509 discloses a method for purifying white phosphorus by stirring white phosphorus in an aqueous phase containing hydrogen peroxide for one hour or longer to reduce the amount of antimony in the white phosphorus to 200 ppb or less. However, in accordance with study results reported by the inventors of the present invention, removal effects of arsenic are not considered.
Meanwhile, PCT Japanese Translation Patent Publication No 2002-516809 discloses a method of removing both arsenic and antimony by bringing liquid crude white phosphorus into contact with an iodine compound such as iodine, iodic acid or iodate and an oxidizing agent such as hydrogen peroxide in the presence of water. However, technologies associated with simultaneous removal of arsenic and antimony are extremely insufficient and a review of a variety of methods is thus required.